Agricultural land use in the Midwestern U.S. is the major source of nitrogen (N) causing recurring hypoxia in the northern Gulf of Mexico. Despite efforts to reduce losses, N export from tile-drained, agricultural watersheds throughout the Corn Belt persists. The use of effective agricultural conservation practices can reduce N loss from fields, yet little is known about how field-scale implementation will translate into watershed-scale reductions in N export. In this study, we used a sampling approach with high spatial and temporal resolution to quantify changes in tile drain load and watershed export of nitrate (NO3 −-N) after planting cover crops on >60% of croppable acres in a small, agricultural watershed. We found that median NO3 −-N losses from tiles draining fields with cover crops were 69–90% lower than tiles draining fields without cover crops during winter/spring. Measured instantaneous flow was the major driver of NO3 −-N losses from tile drains, though results suggest that this relationship differed between tiles with and without cover crops in spring. The signature of cover crops at the field-scale was evident in watershed NO3 −-N export, particularly during times of elevated flows; median daily NO3 −-N exported in elevated flows was 18–22% lower during years with watershed-scale planting of cover crops compared to years without. Nevertheless, changes in watershed NO3 −-N export were smaller than the observed reductions in tile drain loads. Results indicate that tile drain reductions directly reflected the influence of cover crops at the field-scale while watershed export integrated both past and present management, ultimately complicating attempts to distinguish the effect of conservation efforts at larger spatial scales.
- Land use
- Water quality